AIS transmissions

[colbysmith]

Moved from another thread on a different topic...

Me:

[colbysmith]

Some more information I found, that I'm still digesting....


Class A And Class B Automatic Identification System (AIS)
Posted on by Ocean Time Marine

AIS information is divided into two classes – classes A and B – depending on the AIS transponder transmitting the AIS information. These classes are of great importance to the capabilities of your AIS. There is a great difference between the two classes, both in terms of extent, complexity and price.

AIS information from a class A transponder will always be prioritised and, thus, be shown to other ships in the area, whereas AIS information from a class B transponder will not be shown until or if there is room on the AIS channel. AIS of class A In order to avoid that the ships’ AIS systems all speak at the same time, large ships use an AIS system of class A, which is called SOTDMA (Self-Organized TDMA).

An algorithm ensures that the AIS transmitter of a ship first notices how other ships transmit their messages and, subsequently, adjusts its own transmission pattern to that of the others. In case there are more ships fitted with AIS of class A in an area than the capacity of the bandwidth, the system will automatically limit the area of coverage so that the remotest ships in the area are discarded. AIS of class B Small vessels fitted with AIS, such as recreational craft, can use a less expensive AIS station of class B, which transmits less frequently. This system is called CSTDMA (Carrier Sense TDMA). A class B station will listen for a couple of milliseconds to hear whether a large ship is transmitting before it transmits its own message.

Some rather old AIS stations of class A can see only the position, but not the identity of these class B stations – and large ships can choose not to show AIS stations of class B on their displays if their displays become confused because of too many yachtsmen. Thus, the class B information will be shown to other ships in the area only if or when there is room on the AIS channel. On an AIS transponder of class B you can see whether the information transmitted is prioritised and thus displayed on other ships.

[hardee]

Colbly, I think it is important to note that this prioritizing and display is only going on on the vessels AIS display or chart plotter display... not by some "Traffic Control Center" or satellite system. I believe my Watchmate will do that by limiting the number of AIS contacts to a user selectable number up to about 20. Also, when the signals are dense, the farther ones away will drop off the list until they become closer. It is also able to filter approaching compared to non moving or departing vessels from my location or course.

[colbysmith]

Like you Harvey, I also have my AIS on and transmitting whenever I'm moving, or stopped for just a short period. I'll leave it on for a bit when I stop and anchor or dock at night, but then turn it back on a bit before I'm ready to head out. (As opposed to many of the yachts that leave theirs on all the time, even while docked at the marinas...) It is a little disconcerting to know that the larger class A ships can restrict class B targets. After all that's one of the reasons I have my AIS is for all others to see me. I did just send off an email to SiTex to see what they have to say about my transceiver showing an error light at Lake Powell. Will be interesting to get their answer. Colby

[hardee]

Colby,

[hardee]

Again From AIS on Wiki: This is for Class A units, to help understand the "drop out" factor.

[thataway]

Colby, I have seen AIS signal from the tour boat at Powell. There is no way that any entity would or. could block AIS signals at either Yellowstone or Powell.

Could someone theoretically block AIS?--only by transmitting a very strong signal on the frequencies: 161.975 MHz and 162.025 MHz, using a bandwidth of 25 kHz. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted among ships without loss of communication.

GPS can be limited in canyons, but there are several repeaters on Navajo Mountain: VHF Channel 16, National Park service frequencies in VHF range: 171.62500Mhz, The Verizon cell signals, and ham repeaters(146.96000/146.36000; 449.92500 / 444.92500Mhz

US military has been know to blank out GPS for some time, both in the proximity of the bases and of certain ships.

Is it possible to jam AIS--yes, very easy. But for what purpose of reason?--and even if jammed your transmitter should still be transmitting.

It might be possible that there was not an accurate enough position at Powell due to shadowing in canyons, but in Yellowstone that should not happen.

AIS B is picked up by satellites...but the position reports often are outdated. Many larger recreational yachts use AIS A. Nothing in the rules that they are not allowed to use A. The time interval for AIS transmission is .26 milli seconds. This allows 4500 separate vessels to operate in the 40 mile "Range" of AIS without 'interference".

Class A transponders use a system called Self Organised Time Division Multiple Access (SOTDMA), which enables them to negotiate with other Class A transponders to reserve a time slot and book future ones. Class A transmits every 2 seconds. Class B transponders use a different technology called Carrier Sense Time Division Multiple Access (CSTDMA). Class B find points in-between the Class A to transmit every 30 seconds (ideally). Class B is 2 watts, Class A is 12.5 watts output.

To confuse things even more there is now a Class B+ which uses the same SOTDMA program as Class A and transmits at 5 watts every 5 seconds. To be clear the large ships cannot filter out the "transmission of the B" AIS, but he can filter them on his display.

Now Colby and Harey, you need to go out and get the B+ ACR has them for $ 799 , Sitex at $709. Garmin AIS 800 black box at $799. None have a display, but are NMEA 2000 integrated.

[gulfcoast john]

Colby, Harvey, Bob,

Great post, thanks! We have intuitively noticed that when we are navigating really heavy marine traffic areas (esp those with vessel traffic zones like the St Lawrence Seaway, Charleston, SC, Baltimore and Brunswick harbors and Key West Channel type areas) that BIG moving and non-moving targets (like 3 cruise liners tied up by Mallory Square) NEVER go dim or disappear as an AIS target, but other moored and docked boats do appear and disappear in a crowded field. Your research explains why.

Colby, when I had your exact same AIS system symptoms it turned out to be an intermittently bad VHF (shared) antenna that was very hard to diagnose (took over a year) even with a laptop and Garmin diagnostics CD connected to N2K system and Garmin AIS 600 error codes.

Are you sharing the VHF and AIS antenna, or do you have separate antennae for each? B is more desirable if you have the roof space. A 3 ft SS antenna for dedicated AIS use is fine if you don’t much care about AIS traffic that is more than three miles away (I like Digital brand). Harvey, however, has to deal with REALLY fast ferries that don’t ply Southern waters and he may reasonably desire a taller sensor.

While on the subject of how much electronic info is shared constantly and digitally on AIS all on VHF channel 70, we should also applaud the system that simultaneously, all the DSC contacts are also being handled on that one channel all at the same time as AIS signals! If you have heard some laconic deckhand tying up channel 68 or 71 for half an hour complaining about how his parasail customer didn’t give him an adequate tip for his taking their photo....geeez!

If VHF channel 70 gets too busy with all digital info while carrying 1,000 times as much data as ANY other VHF channel, then maybe it’s time to give up some more radio...say, the ‘marine telephone operator’ channels of VHF 24-38 and 84-88! When was the last time you hailed ‘marine telephone operator’ on VHF 16 to make a marine telephone call rather than using your cell phone? 1980 or 1981 for me. When for you?

I don’t want to give up my Air Supply 8 track player tapes, but we think Lew has an 8 track on Cat’s Claw, maybe we can play it at Hontoon.

Cheers!
John

[colbysmith]

[hardee]

I do share one antenna, a Shakespeare short whip, with both a VHF Standard Horizon GX2150, and with the Vision WatchMate AIS through a Vesper SP160
Antenna Splitter. It is an amplified splitter with a 3 db gain.

I did have some trouble initially with the WatchMate, but after sending it back to Rodger's and having them redo the updates, it has performed perfectly. I like having the stand alone display for that one as it makes the ranges and filters very easy access, plus it does give a rough shoreline outline.

The WatchMate Vision is in the upper right corner.


Harvey
SleepyC

[colbysmith]

Both my antennas, including the shared one, are 8' Shakespeare higher quality. The splitter gives the AIS priority, until the VHF it shares with transmits. I believe the splitter also amplifies. Colby

[colbysmith]

Well that was fast! Answer from my AIS manufacturer!


On Dec 21, 2020, at 8:59 AM, Chuck Finch <service@si-tex.com> wrote:

Hi Colby,

I believe the issue you are reporting is due to altitude. Lake Powell is at 3625 and Yellowstone is over 7000 feet. Many GPS units for the marine market are altitude limited as it is expected to be more of a coastal sea level use case. While GPS can be made to be used at altitude ( they use it in planes that fly at 35000 feet!) it is often limited in marine applications to save some cost when designing the product. This usually is verified when a unit will not have a valid position fix even though it sees plenty of satellites and altitude is over 1500-2000 feet. Sorry for the inconvenience.

Best Regards,

Chuck Finch
Service Manager
Si-Tex / Koden
(631) 996-2690 Office
(631) 996-2693 Fax
(631) 276-9068 Mobile
Web: www.si-tex.com
Emial: service@si-tex.com
Chat: (Google Talk): sitexhelp@gmail.com


And then another:

Hi Colby,

I confirmed with the engineers that there is an altitude limit. However, there is a way using ProAIS2 that you can send a command to the unit to changes the limit so it will work at Lake Powell or Yellowstone. Here is what the engineer wrote me:

Yes there is - 500m / 1640 ft above sea level. That is the default altitude for the internal GPS module, and it also applies to the MDA-5. If you want to configure it so that it will work in those locations, sending the following command will allow it to operate at altitudes up to 5000m:

$PSMT,0,3,0x2C75B2FA,1,nvseti "gpsdyn"^2C4^2C3^2C0^2C1,0*76

If you use ProAIS2 to connect to your AIS and then in the “Terminal” tab send the sentence above to the AIS unit it should work fine in the future,

Best Regards,

Chuck Finch
Service Manager
Si-Tex / Koden
(631) 996-2690 Office
(631) 996-2693 Fax
(631) 276-9068 Mobile
Web: www.si-tex.com
Emial: service@si-tex.com
Chat: (Google Talk): sitexhelp@gmail.com

[thataway]

John, The AIS A can be switched so it transmits less frequently when stationary.

I really advocate separate antennas for AIS and VHF, despite the amp (only for incoming signals) and connivance. The AIS antenna should be tuned for the higher frequencies of AIS. (Close to weather channels)

John AIS is NOT on channel 70 (156.525 simplex). That is DSC, no AIS. AIS frequencies as I gave above and are duplex frequencies. There is plenty of bandwidth available to expand AIS (currently channels 1 and 2) to 4 or more channels if this becomes necessary in the future.

Colby, Interesting that Sitex should program the AIS transceiver to only work up to 500'. AIS is used on many lakes world wide which are over 500' high. Also every marine and vehicular GPS I have used, works fine at higher elevations...

Generally GPS receivers can get a reasonable "fix" with 3 good satellites if they are spread out adequately. So you may get GPS signals in a canyon or fjord and yet not get a good fix. We found that in Fords Terror in AK. We ignored the GPS when in steep canyons on Powell. But it would be lost even with still a couple of good satellites showing in the "constellation" view.

Since that is a limit of the Sitex you have, it would probably be a limit in the AIS B+ also. I wonder if other AIS systems also limit the altitude? GPS allows calculation of altitude. I have one which is fairly accurate. Of course in aircraft, you need that to be very accurate.

[hardee]

[colbysmith]